In the fabrication of integrated circuits, one of the more critical procedures is the photolithographic processing. For instance, photolithographic processes in microelectronic devices require precision and reliability in the alignment of the photomasks employed. Accordingly, much work has been done over the years to minimize, as much as possible, the number of alignment steps required.
One technique that has been suggested for reducing photomasking processing time is to provide a multi-layer, multi-density photoresist mask. Such a mask includes a substrate of a transparent material coated in preselected locations with a layer of a semitransparent material and a layer of another material that is opaque. The purpose of such masks of the partially transparent areas is to partially expose a portion of photoresist lying beneath the mask that is being developed. The use of multi-density photoresist masks involves removing the photoresist that is being developed in two or more stages. In particular, the fully exposed photoresist is initially removed and the areas that are exposed in the device being fabricated are then etched. Next, the partially exposed photoresist is removed and different areas are then exposed and can be etched in the fabrication of the desired device.
Examples of suggestions of multi-density photomasks can be found in Cook, et al., "Multi-layer Semitransparent Photomask", IBM Technical Disclosure Bulletin, Volume 15, No. 5, October 1972, pages 1465-1466 and Abolafia, et al., "Dual-Density Mask for Photoresist", IBM Technical Disclosure Bulletin, Volume 19, No. 12, May 1977, page 4539.
The present technology with respect to multi-density photomasks could stand improvement, especially with respect to being able to provide high selectivity to uniformly and reproducibly provide the desired patterns in the different layers without the necessity of resorting to complex and time consuming processing techniques. For instance, the present mask manufacturing methods employ either a subtractive etch process to remove unwanted metal or a lift-off process evaporated through the stencil, and the stencil removed. The problems with these methods for multi-density masks are that the present subtractive process is not selective enough to avoid etching both materials used. On the other hand, the lift-off process requires increased complexity and more process steps and is, thus, relatively expensive.